1. Technical Field
The entire disclosure of Japanese Patent Application Nos: 2006-194910, filed Jul. 14, 2006 and 2006-194911, filed Jul. 14, 2006 and 2007-46263, filed Feb. 26, 2007 are expressly incorporated by reference herein.
The present invention relates to a liquid developer, a method of preparing a liquid developer and an image forming apparatus using the same.
2. Related Art
For a developer used to develop electrostatic latent images formed on latent image carriers, there are available a method of using a dry toner, in which method a toner composed of a colorant such as pigment and the like and a material including a binding resin is used in a dry state; and a method of using a liquid developer in which a toner is dispersed in an electrically insulating carrier liquid (insulating liquid) (see, for example, JP-A-7-152256 (Patent Document 1)).
The method of using a dry toner involves dealing with a toner in the solid state, thus being advantageous in handling; however, the method has problems such as the concern about adverse effects of fine toner particles on human bodies, as well as contamination due to the scattering of toner particles, insufficient homogeneity in toner dispersion, and the like. Furthermore, a dry toner has such problems as that aggregation of particles is likely to occur, it is difficult to sufficiently reduce the size of toner particles, and it is difficult to form toner images with high resolution. Also, when the size of toner particles is made relatively small, the problems caused by fine toner particles as described above become even more significant.
Meanwhile, with regard to the method of using a liquid developer, since aggregation of toner particles in the liquid developer is effectively prevented, it is possible to use fine toner particles and use a binder resin having a low softening point (low softening temperature). As a result, the method of using a liquid developer has features, for example, good reproducibility of images with fine lines, good reproducibility of image tone, and an excellent reproducibility of colors. Furthermore, the method has an excellent feature that image formation is achieved at high speeds. One example of such a method for using the liquid developer is disclosed in Patent Document 1.
When the toner particles used in the liquid developer has a small particle size distribution, the toner images fixed on a recording medium are deemed to have excellent resolution and density properties. Moreover, the toner images thus obtained become less defective and more precise. Additionally, when the liquid developer in a developer vessel is supplied to a coating roller developer vessel, because the toner particles are uniform, a lot of voids are generated between the toner particles and a large quantity of insulating liquid exists between the toner particles. Thus, it is possible to efficiently develop and transfer the toner images. However, the large quantity of the insulating liquid between the toner particles present in the process of the development and transferring operations may remain adhering onto the surfaces of the toner particles upon fixation. Generally, when an image is formed using a known non-volatile mineral oil-based insulating liquid such as hydrocarbon-based or silicone-based insulating liquids, the insulating liquid adhering onto the surfaces of the toner particles may prevent the contact and aggregation of the toner particles upon fixation. For this reason, there was a problem that the toner images are likely to deteriorate by air, light, friction or the like when they are stored for a long period.
Meanwhile, when using the toner particles having a large particle size distribution, it is possible to prevent a decrease in the intensity of fixation. The toner particles having the large particle size distribution include a lot of fine toner particles having a size smaller than an average particle diameter. Accordingly, when the toner particles having the large particle size distribution are used in the liquid developer, the finer toner particles enter into the voids between the larger toner particles in the toner images on the recording medium, and thus the gaps between the toner particles become small. Therefore, only a small amount of the insulating liquid adheres onto the toner particles. As a result, since the amount of the insulating liquid adhering onto the toner particles on the recording medium becomes small, the contact and aggregation of the toner particles can be properly carried out upon fixation. For this reason, the toner images are not likely to deteriorate by air, light, friction or the like when they are stored for a long period. However, since only a small amount of the insulating liquid adheres onto the toner particles even in the process of the developing and transferring operations, there was a problem that the operations of developing and transferring the toner images are not properly carried out. Accordingly, it is difficult to form non-defective and high-resolution toner images on the recording medium.